The current world-wide economy is mainly based on fossil energy sources, such as coal, petroleum, and natural gas. However, these un-renewable fossil energy sources are running out. Since the beginning of the 21st century, the world-wide energy source problem as well as the problems accompanying it, such as the environment pollution and global warming, are emerging and become more and more severe. Solar energy is considered one of the most potential renewable energy sources, due to its advantages such as its wide distribution, great amount, no pollution, cleanliness, safety, and convenient access.
To sufficiently utilize the energy from sunlight irradiation, new materials capable of absorbing sunlight are developed continuously. Among them, inorganic semi-conductive materials have shown broad development and applications, for example in silicon crystal batteries for ground use. However, their applications are limited by their complex manufacturing process and high cost. In order to lower the cost, and expand the application scope, efforts have long been made to seek new alternative semi-conductive materials.
In recent years, organic materials have drawn great attention. For example, since 1992 when N. S. Sariciftci et al. reported the photo-induced electron transfer phenomenon between a conjugated polymer and C60, a great deal of research has been conducted on the use of conjugated polymers in polymeric solar cells, and a rapid progress has been made. A solar cell can directly convert the sunlight energy into electric energy, and is an effective means in utilizing solar energy.
Organic solar cells are a new type of solar cells. Comparing with inorganic semi-conductive materials which have disadvantages such as limited sources, high price, toxicity, complex manufacturing process and high cost, organic solar cells have advantages which cannot be compared by inorganic solar cells, such as broad sources of the materials, diversity and controllability of the structures, low cost, safety and environment-friendliness, simple manufacturing process, light weight, capability of being flexibly manufactured in large areas. Therefore, organic solar cells may be widely used in architecture, illumination, and power generation, and have important prospect in development and applications. However, the energy conversion efficiency of organic solar cells so far is much lower than that of the inorganic solar cells. Accordingly, development of new organic materials is important for increasing the efficiency of organic solar cells and other semi-conductive devices or photo-electric devices.